The present invention relates to electron guns, and particularly to a cathode ray tube electron gun having a split electrode comprising two spaced-apart electrode sections interconnected by a current limiting arc-suppression resistor. The arc-suppression resistor, in the case of an electrical arc will limit the arc current and prevent damaging cascading arcs.
Cascading arcs are defined as a succession of rapidly initiating arcs between electrodes in high field regions of the electrode gun which permit a sufficiently high arc current to pass between electrodes of the electron gun and to subsequently damge the electron gun elements or the associated circuitry.
A conventional cathode-ray tube, for example, a color television picture tube, consists of an evacuated envelope having a neck portion, a faceplate and a funnel portion therebetween. An electron gun is disposed in the neck portion of the envelope, and a tricolor emitting phosphor screen is disposed on the interior surface of the faceplate. A shadow mask is located between the electron gun and the screen, in spaced relation to the screen. The electron gun comprises a plurality of electrodes for focusing and accelerating three electron beams toward the phosphor screen. Typically, several high voltage and low voltage electrodes are serially arranged along the electron beam paths to facilitate the focusing and accelerating of the electron beams. The high voltage electrodes typically operate at an ultor potential of about 30 kilovolts, and the low voltage electrodes typically operate at about 8 to 10 kilovolts or less; however, in some electron guns, an intermediate potential of about 12 kilovolts and a low potential of about 8 kilovolts or less are utilized. A conductive coating having an effective resistance of about 100 ohms is disposed on the interior surface of the funnel portion of the envelope. The interior conductive coating operates at ultor potential. Bulb spacers mounted on the electron gun electrode nearest the phosphor screen contact the interior conductive coating to provide ultor potential to the electron gun. An exterior conductive coating, electrically isolated from the interior conductive coating, is provided on the outside of the funnel to facilitate grounding of the envelope. The interior and exterior conductive coatings on the funnel also serve as a large capacitor which filters the high voltage.
The large voltage difference established between the high voltage and low voltage electrodes in the electron gun creates a possibility of arcing between the electrodes. The possibility of arcing is increased by irregular electrode surfaces, foreign matter in the interelectrode gaps and by misalignment or improper spacing between electrodes. When an arc occurs, the high voltage filter capacitor will, within a few microseconds or less, discharge its stored charge.
Because the instantaneous peak arc currents can approach hundreds or even thousands of amperes in magnitude, great destruction can be caused by such arcs, for example, the external receiver circuitry can be damaged by transient currents and voltages resulting from the arcs. Also, the gun electrodes can be burned or eroded to the point of inoperability, and electrode material may be sputtered onto adjacent surfaces resulting in the creation of leakage paths between tube elements.
In order to reduce tube arcing and to minimize the damage caused thereby, it is common to design cathode-ray tubes with maximum electrode spacings, to minimize field gradients, and to incorporate arc suppression systems into the tube.
Such an arc-suppression structure is disclosed in U.S. Pat. No. 4,345,185 issued to Y. Kobori on Aug. 17, 1982 and discussed by Y. Kobori et al. in their paper entitled, "A Novel Arc-Suppression Technique For Cathode Ray Tubes", presented at the IEEE Chicago Spring Conference on Consumer Electronics, June 19, 1980. The structure disclosed in the Kobori patent requires that a ceramic resistor be connected between the G3 and G5 high voltage electrodes (typically 30 kV) and that another resistor be connected between the low voltage G4 electrode and the stem lead attached thereto. When an arc occurs between adjacent electrodes, the arc current may flow either from the G5 through G3 to the G2 or to the G4. A problem occurs when the initial arc and the plasma generated thereby results in additional arcs, e.g., cascading arcs, between the other electrodes of the electron gun.
Therefore, an arc-suppression system must be able to protect the electron gun not only from the effects of individual arcs but from the effects of a cascading arcs. A number of arc-suppression systems that protect the electron gun from individual arcs and greatly reduces the probability of the occurrence of cascading arcs and the damage therefrom are described in a copending U.S. patent application Ser. No. 424,140 assigned to the same assignee as the present invention and filed on Sept. 27, 1982, by R. Stone and entitled, "Electron Gun Having Arc Suppression Means". The Stone patent application is incorporated by reference herein for the purpose of disclosure. The arc-suppression systems described in the copending Stone patent application require the use of at least two high voltage resistors and in some instances as many as four resistors are used. Since improvements in tube reliability must always be weighed against the cost of such improvements, it is apparent that a low cost arc-suppression system requiring only one resistor is desirable.